The present invention relates to a method for forming an alloy layer upon a substrate metal which is an alloy of aluminum, and more particularly relates to such a method for forming an alloy layer upon a substrate metal, in which the layer is formed by the use of CO.sub.2 laser.
The present invention has been described in Japanese Patent Applications Ser. Nos. 60-178430 (1985) and 60-178431 (1985), both filed by an applicant the same as the entity assigned or owed duty of assignment of the present patent application; and the present patent application hereby incorporates into itself by reference the texts of said Japanese Patent Applications and the claims and the drawings thereof; copies are appended to the present application.
In order to improve the surface qualities of a portion of the surface of a quantity of a substrate metal material such as aluminum alloy, such as for example in order to improve the corrosion resistance, the heat resistance, or the wear resistance of the surface of a member at least largely made of such substrate metal material, it is per se known to be effective to alloy an alloy material into said surface portion of said substrate metal material. One way in which this surface alloying has been performed in the prior art has been to dispose a quantity of the appropriate alloy material on the surface of the substrate metal material, and then to fuse together said alloy material and said substrate metal material by heating by the use of a high energy source such as a laser, a TIG arc, or an electron beam. After the fused portion has solidified, an alloy layer is thus formed, said alloy layer, if the parameters of the process are appropriate and correct, having the appropriately enhanced surface properties.
However, this type of process is fraught with problems which will now be outlined. In the typical event that the heating source which is utilized is a CO.sub.2 type laser of the type which is conventionally used for metal processing and the like, and when the substrate metal material is aluminum alloy which is a typical metal alloy material the surface of parts made from which it is desirable to reinforce, because the aluminum alloy substrate material has a low electrical resistance it is not possible to easily fuse the surface portion of said aluminum alloy substrate material and the alloy material to be added thereto, and it is not in practice possible to form the desired reinforcing surface alloy layer. This is because the layer absorption X is determined by the relationship EQU X=2(mn).sup.1/2
where the electrical resistance of the metal material which is being irradiated by the CO.sub.2 laser is "m" (expressed in micro ohm centimeters) and the frequency of the electromagnetic radiation emitted by the laser is "n". A CO.sub.2 laser emits thermal energy which is electromagnetic radiation in the near infrared wave band with a wave length of about 10.6 microns, and aluminum alloys generally have a low electrical resistance. Thus, even when a substrate piece of aluminum alloy material in the solid form is irradiated by a CO.sub.2 laser which delivers electromagnetic radiation of considerable power, the beam from the CO.sub.2 laser is substantially completely reflected off the surface of the solid substrate aluminum alloy piece, and accordingly the absorption of said laser beam has a very low value, and it is in practice impracticable to heat up the surface of said substrate aluminum alloy piece sufficiently to cause fusing thereof.
If, in order to overcome this phenomenon, an attempt is made to force the alloying process by the simple expedient of increasing the power of the CO.sub.2 laser, then the thermal shock applied not only to the alloy material but also to the substrate aluminum alloy material piece becomes excessive, as a result of which the alloy material becomes scattered and the substrate aluminum alloy material piece becomes pitted and develops relatively large holes. This produces poor alloying. Also, there is the subsidiary problem that the CO.sub.2 laser is required to be of higher output than otherwise, and therefore there is the problem of high capital cost for the alloying process, as well as higher operating cost.
Further, for substantially all aluminum alloys, although when in the solid state the beam from a CO.sub.2 laser is substantially completely reflected as described above, on the other hand when in the molten state said beam from said CO.sub.2 laser is absorbed with relatively high efficacy. Therefore, when forming an alloy surface layer by the method described above using a CO.sub.2 laser, the stage of heating up the solid surface of the substrate with alloy material thereon up to the melting point at which said surface becomes liquid is very important, and has conventionally been carried out by forming a powder of the alloy material. This has been done in order to cause the beam from the CO.sub.2 laser to be reflected diffusely within the alloy material powder layer, thus to be better absorbed thereby, and further in order to reduce the heat capacity of said alloy material layer in order to cause said alloy material to fuse easily. However, if the alloy material is merely made into a powder as described above, then, when the substrate metal material is aluminum alloy as described above, since said substrate metal material has relatively low electrical resistance, the absorption of the beam from the CO.sub.2 laser by said substrate metal material itself is extremely low, as a result of which it is very difficult to form easily the desired alloy surface layer.
Furthermore, considering the question of forming an alloy surface layer upon a metal substrate material in the manner described above from the point of view of metallurgy, in order to form a suitable alloy layer, the ability of the alloy material to form a solid solution with the material of the metal substrate material (the so called "wettability" thereby) and the uniformity of the form of the compound in deposition in the alloy layer are very important. The degree of solid solution of the alloy material can in principle be determined from conventional equilibrium state diagrams and various other research data, and it is desirable in order to form an appropriate alloy layer to make the degree of solid solution of the alloy material relatively high. In a normal type of fusing method such as die welding, the speed of cooling of the fused portion is relatively slow, as a result of which it is difficult to make the degree of solid solution of the alloy material high; but, on the other hand, by using a CO.sub.2 laser as the heat source and by carrying out the method as described above, the molten and fused portion is cooled rapidly by the absorption of heat by the main portion of the matrix material, as a result of which the cooling speed of said fused portion is extremely high and therefore the degree of solid solution of the alloy material is higher than in the cases shown by the equilibrium state diagrams, and thus even materials which are relatively difficult to be alloyed together may be made into a mutual alloy relatively easily, as compared with a normal type of fusing method. Therefore, with the above described type of method using a CO.sub.2 laser as the heat source, the range of choice of materials to be alloyed together may be extended, as compared with the case in which a normal type of fusing method is employed.
However, with regard to the uniformity of the constitution of the alloy layer, since the application of heat and the cooling of the alloyed material are carried out rapidly, and since the degree of agitation of the molten melt portion is inevitably relatively inadequate, a temperature difference arises between the surface layer of said molten melt portion and the portion of said molten melt portion at its bottom and adjacent to the substrate aluminum alloy material, and, particularly in the case that the alloy material is one which easily forms one or more compounds with aluminum, within the deposited alloy layer various compounds tend to be formed. In order to prevent this phenomenon from occurring, or at least to mitigate the effects thereof, it is desirable that such compounds should be distributed relatively evenly in the aluminum which is the principal material making up the aluminum alloy substrate material, and that an element which forms eutectic crystallization should be added to the alloy material; and therefore it is desirable that the alloy material should include a certain quantity of an element which has a relatively small difference of specific gravity from aluminum.